Digital lighting technologies, i.e. illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing different colors, e.g. red, green, and blue, as well as a processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626, incorporated herein by reference.
Lighting fixtures (or “luminaires”) employing a plurality LEDs often have one or more localized bright spots (e.g., localized areas of significantly increased luminance) that are noticeable due to the point source nature of LEDs. For example, LED-based direct-view lighting fixtures implementing LEDs often contain several visible localized bright spots corresponding to the location of the LEDs of the lighting fixture. Also, multi-channel lighting fixtures implementing multiple colors of LEDs of a variety of colors often have one or more localized color spots (e.g., localized areas of visibly different colors) due to the different colors of the LEDs. For example, direct view multi-channel lighting fixtures implementing LEDs often contain several visible localized color spots corresponding to the locations of the various colors of LEDs. These bright spots and/or color spots may provide an undesirable aesthetic appearance when a lighting fixture is directly viewable and/or may provide undesirable lighting characteristics at a location illuminated by a lighting fixture.
Thus, for many LED-based luminaires capable of producing light at particular color points and color temperatures, it is desirable to appropriately mix the light output of such LEDs prior to the light output exiting the LED-based lighting fixture. Appropriate mixing of the LEDs may reduce the presence of any undesired chromatic non-uniformity in the light output of the lighting fixture and provide more desirable light output characteristics. In implementing mixing solutions, many lighting fixtures employ multiple large mixing chambers and/or only provide illumination from a single planar light exit opening. Such configurations may result in an undesirably large mixing solution and/or a mixing solution of limited utility.
Also, various techniques developed for mixing light from LED light sources in the far field, i.e., illuminating a distant surface with light having uniform brightness or color, do not satisfactorily address the color mixing, uniformity, or lit appearance of a direct-view luminaire. Specifically, one important characteristic of a direct-view luminaire is the uniform appearance of the surface that emits light. A uniform appearance is one in which there are no bright or dark areas or color variations in the light, such as greenish or pinkish spots. Preferably, an observer should not be able to distinguish individual light sources (or rows thereof) or discern individual colors (e.g., red, green, or blue) simply by looking at the luminaire.
Color uniformity is important because architects and lighting designers go to great lengths to obscure individual bright spots and color variations on luminaires for aesthetic appeal. For example, fixtures may be installed within a recess (or at a further distance from a wall) to hide scalloping effects and direct glare. The value of a product that creates uniform color on a wall is greatly diminished when the luminaire exhibits prominent color or brightness non-uniformities that have to be hidden using other techniques.
The discrete nature of color LED light sources used in luminaires makes it more difficult to provide a uniform brightness and color for direct-view LED-based luminaires.
Thus, there is a need in the art to provide an LED-based direct-view luminaire producing satisfactory mixing of light output from a plurality of LEDs, such that its light-emitting surface appears substantially uniform in brightness and color, and that may optionally overcome one or more drawbacks with existing mixing solutions.